It appears that there
is an entry in the scripting documentation, however it is only accessible by navigating to the PlusWnd object's Description page and clicking the link from the list there. Same goes for the Contact.ProfileColor property.
Now once you found that page, the answers are straight-forward. You'll find that the BaseColor property is indeed marked as write-only and takes a Win32 RGB value in the form of 0x00bbggrr
(where rr = red, gg = green and bb = blue, in hexadecimal format).
It looks like these properties have been quietly added to Plus!, since I haven't read or heard about it before. I can't think of any interesting use right now but thanks for letting us know anyway!
/ 
